1. Field of the Invention
The present invention relates to an optical fiber for amplifying an optical signal transmitted with an optical fiber and an optical transmission system, and, more particularly, to an optical amplifier which is used for a long-distance optical fiber communication system and the like and amplifies an optical signal with an optical fiber doped with rare earth element (such as erbium, Er).
2. Description of the Related Art
A long-distance optical fiber communication system with optical submarine cables across the ocean passes optical signals through optical fibers, which consists of optical cables, to transmit energy, image/audio signals and the like. Transmission units used in the optical fiber system include light-sending part for sending optical signals, relay part located suitable positions in the optical fiber cable to relay the optical signals from one transmission block to another, and light-receiving part.
An optical amplifier used for the transmission unit amplifiers only the intensity of the optical signal without changing the wavelength of the inputted optical signal. Conventionally, an optical amplifier is applied with a regenerative relay method, in which a sent optical signal is once converted to an electric signal and then the electric signal is re-converted to an optical signal. However, today, a direct optical amplification method is used with the advance of technology for amplifying optical signals directly. An optical amplifier, which amplifiers optical signals directly as mentioned, is usually provided with a optical fiber doped with rare earth element (lanthanoid, actinoid, erbium or the like) as amplification media and a semiconductor laser diode for exciting the rare earth doped optical fiber. In this optical amplifier, a sent optical signal is amplified with the rare earth doped optical fiber excited with the semiconductor laser diode.
FIG. 12 is a block diagram showing one example of a conventional optical amplifier.
The optical amplifier 1, as shown in FIG. 12, is provided with a quartz optical fiber Fib1 used as a transmission medium at the input side, an optical fiber Fib2 at the output side, an erbium(Er) doped optical fiber EDF (erbium doped fiber) used as an amplification medium, an excitation light source LD consisting of a high-power semiconductor laser diode for exciting EDF, a control circuit for controlling the excitation light source LD, an optical fiber Fib3 for transmitting a excitation light outputted from the excitation light source LD, an optical multiplexer WDM (Wavelength Division Multiplexer) for multiplying the excitation light from the excitation light source Ld and the input optical signal from the optical fiber Fib1 at the input side.
In the optical amplifier 1, an input light L1 transmitted through the optical fiber Fib1 at the input side to be a main signal is amplified with the EDF and then outputted through the optical fiber Fib2 at the output side as an output light L2.
The excitation light source LD is, for example, an InGaAsP/InP laser diode of about 1475 nm in oscillation wavelength or an InGaAs laser diode with oscillation wavelength of 980 nm. The excitation light outputted from the excitation light source LD is sent to the optical multiplexer WDM through the optical fiber Fib3.
The optical multiplexer WDM is an optical circuit element for multiplying the excitation light from the excitation light source LD and the input light L1 at the main signal side, of which wavelengths are different, and is provided between the EDF and the optical fiber Fib3 at the output side. The excitation light from the excitation light source LD with the wavelength which is different from that of the input light L1, is introduced to the EDF through the optical Multiplexer WDM.
The erbium (Er) doped in the EDF becomes in a excited state with the introduced excitation light and amplifies a light with a wavelength from 1520 nm to 1570 nm. The input light L1 with a wavelength of 1558 nm is amplified in the EDF and then outputted as the output light L2.
Now, when the excitation light source LD degrades or has a fault because of a long usage or the like, a necessary excitation light can not be obtained. Then, plural excitation light sources LD are provided, and when one excitation light source LD is not usable because of degradation or a fault or the performance thereof lowers, another excitation light source LD is used, that is, a redundant configuration for excitation light sources LD is applied to a optical amplifier.
FIG. 13 is a block diagram showing an optical amplifier 2 with a redundant configuration of excitation light sources.
The optical amplifier 2 shown in FIG. 13 differs from the optical amplifier 1 shown in FIG. 12, and is provided with two excitation light sources L1, L2, an optical coupler Cp for transmitting an excitation light source outputted from one of the excitation light sources L1, L2 to the optical fiber Fib3, an optical fiber Fib4 for transmitting the excitation light from the excitation light source LD1 to the optical coupler Cp, and an optical fiber Fib5 for transmitting the excitation light from the excitation light source LD2 to the optical coupler Cp.
The two excitation light sources LD1, LD2 send the excitation lights to the optical fibers Fib4, Fib5, respectively, extended from the input side of the optical coupler Cp, and are controlled by the control circuit Cn. The control circuit Cn includes a change-over switch, and when a change-over instruction is inputted by an input part not shown, the change-over switch switches the driving current supplied to one excitation light source to anther excitation light source.
The optical coupler Cp is formed by welding with the two optical fiber Fib4, Fib5 side by side, and by cutting an end at the output side of one optical fiber, for example, a top portion of the optical fiber Fib5 connected to the excitation light source Ld2. Though the excitation light is introduced to the optical coupler Cp from one of the optical fibers Fib4, Fib5 extended from the input side, the introduced excitation light is dispersed to another optical fiber at the weld portion, so that it can be transmitted to the optical multiplexer WDM through the optical fiber Fib3 extended from the output side.
Therefore, the excitation light outputted from one of the excitation light sources LD1, LD2 is transmitted to the EDF through the optical coupler Cp and the optical multiplexer WDM.
However, though an optical coupler welded with two optical fibers introduces an excitation light to any optical fiber, the introduced excitation light is diverged to the respective two optical fibers equally at the weld portion in the optical coupler. Therefore, the excitation light diverged to the optical fiber of which output side is cut breaks through the cut portion to the outside, and it causes excess loss. For example, when an optical coupler of 3 dB loss is used, there is a problem in that excess loss more than 3 dB occurs.
And, to minimize the excess loss of the light, it is considered to use a polarization-dependence-type coupler in which a polarization direction is adjusted not to break through a light from a cut portion, however, it is a problem in that the polarization-dependence-type coupler is not suitable for an optical fiber in which a polarization condition varies in accordance with outside factors such as temperature and vibration, and is very expensive.
Further, in an optical amplifier with a redundant configuration of excitation light sources, there is a problem in that an output value of an excitation light introduced to an EDF becomes zero temporarily when an excitation light source to output an excitation light is switched from one light source to another.